Polishing agent and polishing method

ABSTRACT

The present invention relates to a polishing agent for polishing a surface to be polished of an object to be polished, the polishing agent including: first silicon oxide fine particles having an average primary particle size of 5 to 20 nm; second silicon oxide fine particles having an average primary particle size of 40 to 110 nm; and water, in which a ratio of the first silicon oxide fine particles to a total amount of the first silicon oxide fine particles and the second silicon oxide fine particles is from 0.7 to 30% by mass.

TECHNICAL FIELD

The present invention relates to a polishing agent for polishing a surface to be polished of an object to be polished, and a polishing method. More particularly, the invention relates to a polishing agent which makes it possible to perform high-speed polishing and is excellent in stability at the time when used for a long period of time, in polishing a surface to be polished of an object to be polished, and a polishing method using the same.

BACKGROUND OF THE INVENTION

Techniques for producing and processing wafers of compound single-crystals such as sapphire (α-Al₂O₃), silicon carbide (SiC) and gallium nitride (GaN), as base materials for LEDs or power devices which are expected to greatly glow in the future, are attracting attention. On each of these substrates, a crystalline thin film of GaN or the like is formed to integrate a device, so that a crystallographically less defective, high-quality surface is regarded to be important. In order to obtain the less defective, high smooth surface, a chemical mechanical polishing (hereinafter also referred to as CMP) technique is drawing attention. However, all of sapphire, SiC and GaN have very high hardness and also have high chemical stability, so that it is difficult to perform polishing with high efficiency while ensuring quality, particularly in polishing in a final stage which determines quality, and there has been a problem that a polishing step becomes very long.

In many cases, silicon oxide fine particles have hitherto been used for final polishing which determines quality of these single-crystal substrates. Some attempts to increase the polishing efficiency (removal rate) by using the silicon oxide fine particles have hitherto been made, and it has been proposed to increase the abrasive concentration (see Non-Patent Document 1), to mix two or more abrasives different in particle size at a predetermined ratio (see Patent Documents 1 and 2), to increase the polishing pressure/rotation speed, and the like.

BACKGROUND ART PATENT DOCUMENT

Patent Document 1: Japanese Patent No. 4231632

Patent Document 2: Japanese Patent No. 4253141

Non-Patent Document

Non-Patent Document 1: “Scratch-free Dielectric CMP Process with Nano-colloidal Ceria Slurry”, pp. 31-34, International Conference on Planarization/CMP Technology, Nov. 19-21, 2009

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

However, in polishing of the single-crystal substrates using the silicon oxide fine particles, the polishing agent is generally used in cycles, and it is necessary to consider stability at the time when used for a long period of time. When the abrasive concentration is increased, the abrasive is aggregates by use, and the polishing efficiency becomes liable to substantially decrease. There is therefore a problem that the stability at the time when the polishing agent is used for a long period of time is deteriorated. Also in mixing of the abrasives, at mixing ratios which have hitherto been proposed, it is cited as a problem that not only an effect of improving the removal rate is limited, but also the stability of the polishing agent at the time when used for a long period of time is deteriorated. Further, when the polishing conditions are made strict, the removal rate can be increased. However, a problem of a wafer shape or polishing defects such as scratches becomes liable to be induced.

The invention has been made in order to solve the above-mentioned problems, and an object thereof is to provide a polishing agent which polishes a surface to be polished of an object to be polished at a higher rate and is also excellent in stability at the time when used for a long period of time, and a polishing method.

Means for Solving the Problems

The invention provides a polishing agent for polishing a surface to be polished of an object to be polished, which has the following constitutions.

[1] A polishing agent for polishing a surface to be polished of an object to be polished, the polishing agent comprising: first silicon oxide fine particles having an average primary particle size of 5 to 20 nm; second silicon oxide fine particles having an average primary particle size of 40 to 110 nm; and water, wherein a ratio of the first silicon oxide fine particles to a total amount of the first silicon oxide fine particles and the second silicon oxide fine particles is from 0.7 to 30% by mass.

[2] The polishing agent according to [1], wherein both the first silicon oxide fine particles and the second silicon oxide fine particles are colloidal silica.

[3] The polishing agent according to [1] or [2], wherein the ratio of the first silicon oxide fine particles to the total amount of the first silicon oxide fine particles and the second silicon oxide fine particles is from 1 to 10% by mass.

[4] The polishing agent according to any one of [1] to [3], wherein the second silicon oxide fine particles have an average primary particle size of 45 to 100 nm.

[5] The polishing agent according to any one of [1] to [4], wherein the first silicon oxide fine particles have an average primary particle size of 5 to 15 nm.

[6] The polishing agent according to any one of claims [1] to [5], wherein the object to be polished is a single-crystal substrate having a revised Mohs hardness of 10 or more.

The invention further provides a method for polishing a surface to be polished of an object to be polished, which has the following constitutions.

[7] A polishing method comprising: supplying the polishing agent according to any one of claims [1] to [6] to a polishing pad; and bringing the polishing pad into contact with a surface to be polished of an object to be polished to perform polishing by relative movement between the polishing pad and the surface to be polished.

[8] The polishing method according to claim [7], wherein an operation of recovering the polishing agent supplied to the polishing pad and used for polishing, and of supplying again the recovered polishing agent to the polishing pad is repeated, thereby using the polishing agent in cycles.

Advantage of the Invention

According to the polishing agent of the invention and the polishing method using the same, the surface to be polished of the object to be polished can be polished at a high rate, and further, the abrasive can be stably used for a long period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing an example of a polishing machine which can be used for a polishing method of the invention.

MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention will be described below.

[Polishing Agent]

The polishing agent according to the invention is a polishing agent for polishing a surface to be polished of an object to be polished and contains first silicon oxide fine particles having an average primary particle size of 5 to 20 nm, second silicon oxide fine particles having an average primary particle size of 40 to 110 nm and water. The ratio of the above-mentioned first silicon oxide fine particles to the total amount of the above-mentioned first silicon oxide fine particles and second silicon oxide fine particles is from 0.7 to 30% by mass.

In the polishing agent of the invention, the first silicon oxide fine particles and the second silicon oxide fine particles are used as polishing abrasives. In the polishing agent of the invention, the average primary particle size of the first silicon oxide fine particles and the average primary particle size of the second silicon oxide fine particles are adjusted to the above-mentioned ranges, respectively. By introducing them into the polishing agent at the above-mentioned compounding ratio, the first silicon oxide fine particles having a smaller particle size which are present in a small amount among the second silicon oxide fine particles having a larger particle size which occupy a large part thereof at the above-mentioned compounding ratio as the polishing abrasives increases friction force between the substrate and the abrasive. Accordingly, the high removal rate is obtained. Further, the presence of the first silicon oxide fine particles having a smaller particle size in a small amount as the above-mentioned compounding ratio together with the second silicon oxide fine particles having a larger particle size also contributes to an improvement in dispersion stability in a dispersion medium such as water to obtain the stability at the time of long-term use.

(1) First Silicon Oxide Fine Particles and Second Silicon Oxide Fine Particles

In the polishing agent of the invention, the same silicon oxide fine particles except for having different average primary particle sizes can be used as the first silicon oxide fine particles and the second silicon oxide fine particles, and ones produced by various known methods can be used as both fine particles. Examples thereof include silicon oxide fine particles such as colloidal silica obtained by subjecting sodium silicate or fumed silica obtained by vapor-phase synthesis of silicon tetrachloride in flame of oxygen and hydrogen to ion exchange or desalination after neutralization, or colloidal silica obtained by hydrolysis of a silicon alkoxide in a liquid phase. Of these, in the polishing agent of the invention, colloidal silica in which sodium silicate is used as a starting material is more preferred, from the viewpoint of the diversity of varieties.

The average primary particle size of the first silicon oxide fine particles contained in the polishing agent of the invention is from 5 to 20 nm, as described above. However, it is preferably from 5 to 15 nm, and more preferably from 7 to 13 nm.

There is a concern that the first silicon oxide fine particles of less than 5 nm cannot stably exist, whereas when the first silicon oxide fine particles exceeding 20 nm are used, there is a possibility that the preferred removal rate is not obtained.

Further, the average primary particle size of the second silicon oxide fine particles contained in the polishing agent of the invention is from 40 to 110 nm, as described above. However, it is preferably from 45 to 100 nm. When the second silicon oxide fine particles exceeding 100 nm are used, there is a concern that the surface accuracy of the surface to be polished of the object to be polished is deteriorated. When the second silicon oxide fine particles of less than 40 nm are used, there is a possibility that the preferred removal rate is not obtained.

Incidentally, in this specification, the average primary particle size of the silicon oxide fine particles is one obtained by converting the specific surface area measured by a nitrogen adsorption BET method to the diameter of spherical particles.

Further, the compounding ratio of the above-mentioned first silicon oxide fine particles and second silicon oxide fine particles in the polishing agent of the invention is the compounding ratio at which the ratio of the first silicon oxide fine particles to the total amount of the first silicon oxide fine particles and the second silicon oxide fine particles becomes 0.7 to 30% by mass, as described above. This compounding ratio is preferably from 1 to 10% by mass, and more preferably from 3 to 10% by mass.

It is preferred that the content of the first silicon oxide fine particles and the second silicon oxide fine particles in the polishing agent of the invention is appropriately set as the total content of the first silicon oxide fine particles and the second silicon oxide fine particles within the range of 10 to 50% by mass based on the total mass of the polishing agent, in view of the removal rate, uniformity, material selectivity, dispersion stability and the like. When the total content of the first silicon oxide fine particles and the second silicon oxide fine particles is less than 10% by mass based on the total mass of the polishing agent, the sufficient removal rate is not sometimes obtained, whereas when it exceeds 50% by mass, an improvement of the removal rate corresponding to an increase in abrasive concentration is not observed, and further, the viscosity of the polishing agent excessively increases, or gelation of the polishing agent is enhanced, in some cases.

Further, the total content of the first silicon oxide fine particles and the second silicon oxide fine particles in the polishing agent of the invention is more preferably within the range of 15 to 30% by mass based on the total mass of the polishing agent.

(2) Water

Water contained in the polishing agent of the invention is a medium for dispersing the above-mentioned first silicon oxide fine particles and second silicon oxide fine particles as polishing abrasives, and for dispersing and dissolving other optional components added as needed. With respect to water, there is no particular limitation. However, from influences to other compounding components, contamination of impurities and influences to the pH and the like, pure water or deionized water is preferred. Water has a function of controlling fluidity of the polishing agent, so that the content thereof can be appropriately set according to intended polishing characteristics such as the removal rate and smoothing characteristics.

In the polishing agent of the invention, water is preferably contained within the range of 40 to 90% by mass based on the total mass of the polishing agent. When the content of water is less than 40% by mass based on the total mass of the polishing agent, the viscosity of the polishing agent increases to impair the fluidity in some cases, whereas when it exceeds 90% by mass, the concentration of the above-mentioned first silicon oxide fine particles and second silicon oxide fine particles as the polishing abrasives is sometimes decreased to fail to obtain the sufficient removal rate.

(3) Preparation of Polishing Agent and Optional Components

The polishing agent of the invention can be prepared by weighing the first silicon oxide fine particles and second silicon oxide fine particles of the above (1) and water of (2) which are contained as essential components, for example, so as to achieve the above-mentioned compounding amount, and mixing them.

Here, when colloidal silica is used as both the first silicon oxide fine particles and the second silicon oxide fine particles, the polishing agent of the invention can be prepared only by mixing colloidal silica containing the above-mentioned first silicon oxide fine particles and colloidal silica containing the above-mentioned second silicon oxide fine particles at a desired ratio and appropriately diluting the mixture with water, because colloidal silica is supplied in a state where the silicon oxide fine particles are previously dispersed in water.

Incidentally, in addition to the essential components of the above (1) and (2), optional components contained in normal polishing agent for chemical mechanical polishing may be contained in the polishing agent of the invention within the range not impairing the above-mentioned effects of the invention.

(4) Object to be Polished

The polishing agent of the invention is a polishing agent for polishing the surface to be polished of the object to be polished, and the object to be polished is not particularly limited. Specific examples thereof include glass substrates, silicon wafers, semiconductor device wiring substrates, compound single-crystal substrates and the like. Of these, the polishing agent of the invention can achieve a greater effect in polishing the compound single-crystal substrates, and particularly, effects of higher-rate polishing and longer-term stable use can be largely expected by use thereof for the single-crystal substrates having a revised Mohs hardness of 10 or more.

Specific examples of the above-mentioned single-crystal substrates having a revised Mohs hardness of 10 or more include sapphire (α-Al₂O₃) substrates (hardness: 12), silicon carbide (SiC) substrates (hardness: 13), gallium nitride (GaN) substrates (hardness: 13) and the like. Of these, the polishing agent of the invention can be preferably used particularly for polishing of the sapphire substrates.

[Polishing Method]

As a method for polishing the surface to be polished of the object to be polished using the polishing agent of the invention, preferred is a polishing method of bringing a polishing pad into contact with the surface to be polished of the object to be polished while supplying the polishing agent to the polishing pad to perform polishing by relative movement between the polishing pad and the surface to be polished.

In the above-mentioned polishing method, a conventionally known polishing machine can be used as a polishing machine. An example of a polishing machine which can be used in embodiments of the invention and in which the polishing agent is used in cycles is shown in FIG. 1 and described below. However, the polishing machine used in the embodiments of the invention should not be construed as being limited to one having such a structure.

This polishing machine 10 is provided with a polishing head 2 for holding an object to be polished 1, a polishing platen 3, a polishing pad 4 stuck onto a surface of the polishing platen 3, a tank 8 for storing an polishing agent 5 and a polishing agent supply pipe 6 for supplying the polishing agent 5 from the tank 8 to the polishing pad 4 using an polishing agent supply pump 7. The polishing machine 10 is constituted in such a manner that a surface to be polished of the object to be polished 1 held by the polishing head 2 is brought into contact with the polishing pad 4, while supplying the polishing agent 5 from the polishing agent supply pipe 6, and that the polishing head 2 and the polishing platen 3 are relatively rotated to perform polishing.

Using such a polishing machine 10, the surface to be polished of the object to be polished 1 can be polished. Here, the polishing machine 10 is a polishing machine which polishes one surface of the object to be polished as the surface to be polished. For example, however, it is also possible to polish the surfaces to be polished (both surfaces) of the object to be polished, using a double-sided simultaneous polishing machine in which the same polishing pads as used in the polishing machine 10 are disposed on upper and lower surfaces of the object to be polished.

The polishing head 2 may perform not only rotation movement but also linear movement. Further, the polishing platen 3 and the polishing pad 4 may have a size equivalent to or less than that of the object to be polished 1. In that case, it is preferred to relatively move the polishing head 2 and the polishing platen 3, thereby making it possible to polish the entire surface of the surface to be polished of the object to be polished 1. Furthermore, the polishing platen 3 and the polishing pad 4 may not perform rotation movement, but may move, for example, in one direction by a belt system.

Although there is no particular limitation on polishing conditions of such a polishing machine 10, it is also possible to more increase the polishing pressure and to improve the removal rate by applying a load to the polishing head 2 to press it against the polishing pad 4. The polishing pressure is preferably from about 10 to 50 kPa, and from the viewpoints of uniformity of the removal rate in the surface to be polished of the object to be polished 1, smoothing and prevention of polishing defects such as scratches, it is more preferably from about 10 to 40 kPa. The number of rotations of the polishing platen 3 and the polishing head 2 is preferably from about 50 to 500 rpm, but is not limited thereto. Further, the amount of the polishing agent 5 supplied is appropriately adjusted and selected by a constituent material of the surface to be polished, a composition of the polishing agent, the above-mentioned polishing conditions and the like. However, for example, when a wafer having a diameter of 50 mm is polished, the amount thereof supplied is preferably from approximately 5 to 300 cm³/min.

As the polishing pad 4, there can be used one made of a usual nonwoven fabric, a foamed polyurethane, a porous resin, a non-porous resin or the like. Further, in order to accelerate the supply of the polishing agent 5 to the polishing pad 4 or to allow a certain amount of the polishing agent 5 to stay in the polishing pad 4, lattice-shaped, concentric or helical grooves may be processed on a surface of the polishing pad 4.

Furthermore, a pad conditioner may be brought into contact with the surface of the polishing pad 4 to perform polishing while conditioning the surface of the polishing pad 4, as needed.

In addition, the polishing machine 10 shown in FIG. 1 is constituted in such a manner that it has a recovery unit (not shown) for recovering the polishing agent 5 used for polishing from the polishing pad 4, and that the polishing agent 5 recovered is transferred to the tank 8. The polishing agent 5 which has returned to the tank 8 is supplied again to the polishing pad 4 through the polishing agent supply pipe 6 using the polishing agent supply pump 7. In this way, the polishing agent 5 is used in cycles.

Incidentally, in the polishing method of the invention, the polishing agent supplied to the polishing pad is recovered in the same manner as described above after used for polishing. However, it is also possible to use a polishing machine having a so-called no-return constitution in which the abrasive is discarded for every polishing use.

The polishing method in which the polishing agent is used in cycles is preferred as compared to the polishing method in which the polishing agent is discarded for every polishing use, because the consumption thereof can be reduced. However, with the progress of polishing, the polishing agent is contaminated with components of the material to be polished by polishing, so that there has been a problem that the conventional polishing agent is liable to bring about aggregation or gelation of polishing abrasives, which induces clogging of the pad to gradually decrease the removal rate. According to the polishing agent of the invention, the gelation or the aggregation due to the contamination of the components of the polished material, which are generated by the above-mentioned polishing, is difficult to occur, and the decrease in the removal rate at the time of cyclic use is suppressed.

That is to say, the polishing agent of the invention is characterized in that the initial removal rate is high, and that the decrease in the removal rate at the time of use in a cyclic system is suppressed. Thereby, not only the efficiency of the polishing step is improved, but also the downtime is shortened by a reduction in the consumption of the polishing agent or a decrease in dressing or flashing frequency of the pad. Further, this also leads to a reduction in the consumption of the pad, and the polishing step can be efficiently performed. It can therefore be said that the significance of the invention to improvement in mass production of various devices is very large.

EXAMPLES

The invention will be described below with reference to examples, but the invention should not be construed as being limited to the following description. Examples 1 to 6 are working examples of the invention, and Examples 7 to 12 are comparative examples.

Example 1

Colloidal silica having an average primary particle size of 10 nm (an aqueous dispersion of first silicon oxide fine particles having a solid concentration of 40% by mass) as first silicon oxide fine particles and colloidal silica having an average primary particle size of 80 nm (an aqueous dispersion of second silicon oxide fine particles having a solid concentration of 40% by mass) as second silicon oxide fine particles are mixed at such a ratio that the compounding ratio of the first silicon oxide fine particles to the total amount of the first silicon oxide fine particles and the second silicon oxide fine particles becomes 1% by mass, followed by thorough stirring. Ion-exchange water was added to a resulting mixed liquid in such a manner that the total amount of the first silicon oxide fine particles and the second silicon oxide fine particles based on the total mass of the polishing agent to be finally obtained, namely based on the total mass of the total amount of the first silicon oxide fine particles and the second silicon oxide fine particles and water, becomes 20% by mass, to prepare a polishing agent. In the resulting polishing agent, the first silicon oxide fine particles and the second silicon oxide fine particles are abrasive components.

With respect to the abrasive components composed of the first silicon oxide fine particles and the second silicon oxide fine particles, respectively, in the polishing agent obtained in Example 1 described above, the average primary particle size and the compounding ratio of the respective silicon oxide fine particles are shown in Table 1. In Example 1 and all Examples (2 to 12) shown below, the existing ratio of abrasive components:water in polishing agents is 20:80 (mass ratio).

Incidentally, the average primary particle size of the silicon oxide fine particles compounded in the polishing agent is a value obtained by measuring the specific surface area by the nitrogen adsorption BET method. All the average primary particle sizes of the silicon oxide fine particles used in Examples 2 to 12 described below are values obtained by performing the measurement in the same manner.

Examples 2 to 12

In the same manner as in Example 1, first silicon oxide fine particles and second silicon oxide fine particles each having an average primary particle size shown in Table 1 were compounded as abrasive components so as to give a composition shown in Table 1, and further, water was added thereto in such a manner that the total amount of the first silicon oxide fine particles and the second silicon oxide fine particles based on the total mass of the polishing agent, namely the compounding amount of the abrasive components, becomes 20% by mass. Thus, polishing agents of Examples 2 to 12 were prepared. Incidentally, all the silicon oxide fine particles used were colloidal silica.

[Evaluation]

Polishing characteristics of the polishing agents of Examples 1 to 12 obtained above were evaluated by the following methods.

As the evaluation of the polishing characteristics, there were performed (1) evaluation of the removal rate at the time when the polishing agent was used in the no-return system, and (2) evaluation of continuity of the removal rate at the time when the polishing agent was used in the cyclic system.

<Material to be Polished>

In both evaluation (1) and evaluation (2), a 2-inch wafer of a single-crystal sapphire substrate (manufactured by Shinkosha Co., Ltd., (0001) plane, the thickness of the substrate: 420 μm) was used as a material to be polished.

<Polishing Method>

As a polishing machine, there was used a bench polishing machine manufactured by Speedfam Co., Ltd. As polishing pads, there were used (1) K-groove of single layer IC 1000 (no-return use) and (2) SUBA 800-XY-groove (cyclic use) (both manufactured by Nitta Haas Incorporated). Before the test, conditioning was performed using MEC100-PH3.5L (manufactured by Mitsubishi Material Corporation) and a brush.

The polishing was performed under conditions where the supply rate of the polishing agent was (1) 10 cm³/min (no-return use) and (2) 100 cm³/min (cyclic use), the number of rotations of a polishing platen was 100 rpm, the polishing pressure was 5 psi, namely 34.5 kPa, and the polishing time was (1) 30 minutes (no-return use) and (2) 60 minutes (cyclic use). Further, when the polishing agent was used in cycles, the above-mentioned sapphire substrate was changed every 60 minutes, and the polishing was continuously performed without conducting any pad conditioning in the course thereof.

<Removal Rate>

The removal rate was evaluated by the amount of change in thickness of the substrate per unit time (μm/hr). Specifically, with respect to each of the single-crystal sapphire substrates used for the evaluation of the above (1) and (2), the mass of the unpolished substrate having a known thickness and the mass of the substrate after polished for each period of time were measured, and the mass change was determined from the difference therebetween. Further, the change in thickness of the substrate per period of time determined from the mass change was calculated using the following formulas.

(Calculation Formulas of Removal Rate (V))

Δm=m0−m1

V=Δm/m0×T0×60/t

(in the formula, Δm (g) represents the mass change between before and after polishing, m(0) (g) represents the initial mass of the unpolished substrate, m1(g) represents the mass of the substrate after polished, V represents the removal rate (μm/hr), T0 represents the substrate thickness (μm) of the unpolished substrate, and t represents the polishing time (min).

<Initial Removal Rate>

First, with respect to the polishing agents of Examples 1 to 12, the removal rate under no-return (non-cyclic use) polishing agent conditions was measured and calculated as the initial removal rate according to the above-mentioned polishing method (1). Incidentally, when the initial removal rate of the polishing agent prepared in Example 7, in which only the second silicon oxide fine particles having an average primary particle size of 80 nm was used as the abrasive, was taken as 1.00, the ratio at that time was determined, and the initial removal rate was represented thereby. The results thereof are shown in Table 1.

TABLE 1 Abrasive Component Composition First Colloidal Silica Second Colloidal Silica compounding compounding Evaluation Average Primary Amount Average Primary Amount Initial Example Particle Size [nm] [mass %] Particle Size [nm] [mass %] Removal Rate* Example 1 10 1 80 99 1.12 Example 2 10 5 80 95 1.27 Example 3 10 10 80 90 1.24 Example 4 10 25 80 75 1.20 Example 5 17 5 80 95 1.18 Example 6 10 5 54 95 1.15 Example 7 (Comparative Example) 10 0 80 100 1.00 Example 8 (Comparative Example) 10 0.5 80 99.5 1.02 Example 9 (Comparative Example) 10 50 80 50 1.07 Example 10 (Comparative Example) 10 100 80 0 0.94 Example 11 (Comparative Example) 27 10 80 90 1.01 Example 12 (Comparative Example) 10 5 120 95 0.68 *Ratio of the removal rate at the time when the removal rate of Example 7 was taken as 1.00

<Continuity of Removal Rate>

Then, continuity of the removal rate at the time when the polishing agent was used in cycles was evaluated by the following method. The polishing method was a method according to the above (2). The continuity of the polishing agent at the time of cyclic use was evaluated by the accumulated removal amount of the sapphire substrate at the time when the polishing was performed until the removal rate measured and calculated every 60 minutes was decreased by 15% compared to the initial removal rate (removal rate for 60 minutes from the initiation of the polishing). Taking the accumulated removal amount with the polishing agent of Example 7 as 1.00, the continuity of the abrasive was represented by the ratio thereof. When the value is larger than 1.00, it shows that maintenance of the removal rate is better than that of the polishing agent of Example 7.

The term “gel” as used herein is a kind of dispersion system and a colloid of a liquid dispersion medium such as a sol, but means a state where it has high viscosity due to a network of a dispersed material to lose fluidity, resulting in becoming solid as the entire system, different from the sol.

TABLE 2 Abrasive Component Composition First Colloidal Silica Second Colloidal Silica compounding compounding Evaluation Average Primary Amount Average Primary Amount Removal Rate Example Particle Size [nm] [mass %] Particle Size [nm] [mass %] Continuity** Example 1 10 1 80 99 1.09 Example 2 10 5 80 95 1.42 Example 3 10 10 80 90 1.20 Example 4 10 25 80 75 1.08 Example 7 (Comparative Example) 10 0 80 100 1.00 Example 8 (Comparative Example) 10 0.5 80 99.5 1.00 Example 9 (Comparative Example) 10 50 80 50 Gelled Example 10 (Comparative Example) 10 100 80 0 Gelled **Ratio of the accumulated removal amount at the time when the accumulated removal amount of Example 7 was taken as 1.00

As will be seen from Tables 1 and 2, the polishing agents each containing the first silicon oxide fine particles and the second silicon oxide fine particles each having an average primary particle size defined by the invention at a compounding ratio defined by the invention have a higher removal rate and good continuity of the removal rate during use, namely excellent long-term use stability, compared to the polishing agents of Comparative Examples.

While the invention has been described in detail with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Incidentally, the present application is based on Japanese Patent Application No. 2010-156536 filed on Jul. 9, 2010, and the contents are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

According to the invention, it becomes possible to perform high-speed polishing of object to be polished which is polished, particularly surface to be polished of compound single-crystal substrates having high hardness, such as sapphire (α-Al₂O₃) substrates, silicon carbide (SiC) substrates and gallium nitride (GaN) substrates, and it becomes possible to improve long-term use stability of the polishing agent. Thereby, the invention can contribute to improvement in productivity of these substrates.

Description of Reference Numerals and Signs

1 . . . Object to be polished,

2 . . . Polishing head,

3 . . . Polishing platen,

4 . . . Polishing pad,

5 . . . Polishing agent,

6 . . . Polishing agent supply pipe,

7 . . . Polishing agent supply pump,

8 . . . Tank,

10 . . . Polishing machine 

1. A polishing agent for polishing a surface to be polished of an object to be polished, the polishing agent comprising: first silicon oxide fine particles having an average primary particle size of 5 to 20 nm; second silicon oxide fine particles having an average primary particle size of 40 to 110 nm; and water, wherein a ratio of the first silicon oxide fine particles to a total amount of the first silicon oxide fine particles and the second silicon oxide fine particles is from 0.7 to 30% by mass.
 2. The polishing agent according to claim 1, wherein both the first silicon oxide fine particles and the second silicon oxide fine particles are colloidal silica.
 3. The polishing agent according to claim 1, wherein the ratio of the first silicon oxide fine particles to the total amount of the first silicon oxide fine particles and the second silicon oxide fine particles is from 1 to 10% by mass.
 4. The polishing agent according to claim 1, wherein the second silicon oxide fine particles have an average primary particle size of 45 to 100 nm.
 5. The polishing agent according to claim 1, wherein the first silicon oxide fine particles have an average primary particle size of 5 to 15 nm.
 6. The polishing agent according to claim 1, wherein the object to be polished is a single-crystal substrate having a revised Mohs hardness of 10 or more.
 7. A polishing method comprising: supplying the polishing agent according to claim 1 to a polishing pad; and bringing the polishing pad into contact with a surface to be polished of an object to be polished to perform polishing by relative movement between the polishing pad and the surface to be polished.
 8. The polishing method according to claim 7, wherein an operation of recovering the polishing agent supplied to the polishing pad and used for polishing, and of supplying again the recovered polishing agent to the polishing pad is repeated, thereby using the polishing agent in cycles. 